Studies have shown that at least one type of nanostructure, carbon nanotubes (CNTs), may be able to replace copper in both vertical and horizontal microelectronic interconnects (Naeemi and Meindl, IEEE Trans. Electron Devices 54(1):26-37, 2007). In particular, it has been theoretically shown that for global interconnects, bundles of single-wall nanotubes (SWNTs) or multi-wall nanotubes (MWNTs) can potentially replace copper wires by allowing the use of smaller interconnect dimensions while keeping delay and crosstalk noise constant, thus increasing the bandwidth density of global interconnects.
In addition, while copper is prone to electromigration and boundary scattering at emerging linewidths of less than 100 nm, CNTs are resistant to electromigration, have μm electron mean free path, and can handle substantially higher current densities up to 109 A/cm2. Calculations have also shown that CNTs can decrease switching energy consumption, and GHz operation of a single large diameter multi-wall CNT (MWCNT) as a horizontal interconnect has been experimentally realized (Close et al., Nano Lett. 8(2):706-709, 2008).
However, various challenges can arise when applying CNTs in interconnect technology. Some of these challenges include: 1) providing for horizontal orientation of CNT bundles on a chip; 2) providing a high packing fraction or density of CNTs; 3) providing CNT growth conditions such as temperature, pressure, and gas composition that are compatible with CMOS processing over wafer-scale areas; and 4) providing low contact resistance by assuring contact to all graphene shells (walls) of all tubes. Further, as key performance parameters (such as mean free path, number of conduction channels, etc.) depend on CNT length and diameter, a fabrication strategy would preferably facilitate tunability of CNT diameter as well as build interconnects from continuous parallel CNTs.
Researchers have sought to fabricate horizontally-aligned CNTs by direct growth on substrates (e.g., alignment by crystallographic interactions or gas flows), possibly followed by transfer printing. But sufficiently high CNT densities have not been achieved using these methods, and multi-layer approaches such as repeated transfer printing of single layers of CNTs require an impractical number of steps.
One method of attempting to obtain high density horizontally-aligned CNTs is capillarity-driven densification by controlled dipping of patterned sections of vertically-aligned CNTs (VA-CNTs) in solvents such as IPA or acetone (Hayamizu et al., Nature Nanotechnology 3:289-294, 2008). By engineering the catalyst and the dipping/drawing motion, “CNT wafers” consisting of horizontally aligned overlapping arrays of CNTs have been manufactured and used in device fabrication. The density that can be achieved using this method is limited by the zipping force of the solvent that results from the liquid surface tension and the contact angle between the solvent used and the CNTs.
Another method includes obtaining a CNT film from a “CNT carpet” by shearing the top of VA-CNT arrays, using a thin sheet of foil to lay the arrays down without disturbing their alignment, and compressing the CNT film covered by the foil using a roller. Finally, the CNT film may be detached from the foil and the growth substrate and transferred to different materials host substrates using a dry peel and place method (Pint et al., ACS Nano 2(9):1871-1878, 2008).
Another method includes manufacturing “CNT papers” by pushing a microporous membrane against a CNT forest by means of a cylinder having diameter much larger than the CNT forest height. The effect of the rolling motion of the cylinder on the CNT forest is compared to dominos pushing one another over where it is hypothesized that CNTs are sliding on each other to achieve the final aligned CNT film structure. The porous membrane (with the CNTs sticking to it) is peeled off of the growth substrate and ethanol is spread on the membrane to release the CNT paper (Wang et al., Nanotechnology 19:1-6, 2008).
Rolling out of vertical CNTs using a large diameter roller to obtain horizontally aligned CNT structures is also disclosed in U.S. Pat. No. 7,514,116 B2.